This invention relates to the field of treatment of glass objects for increasing the mechanical strength thereof and, more particularly, to a novel method for increasing service strength through the reforming of surface defects or flaws.
In order to promote safety and serviceability, extensive research has been devoted to the objective of increasing the strength of glass objects, and most especially, their resistance to damage and failure under conditions of practical usage and handling. Since glass is widely used in packaging, particularly in containers for beverages, including carbonated beverages, much of the research has been devoted to strengthening techniques adapted to the configuration, service and handling conditions encountered with glass containers.
It has long been recognized that glass is intrinsically a very strong material. Based on the energy of the Si-O bond, the theoretical intrinsic strength of silica glass has been estimated at 2,000,000 psi or greater. Because of the effect of surface imperfections, however, the nominal tensile strength of annealed glass is commonly only about 7,000 psi. Such surface defects act as stress multipliers which can raise a nominally applied stress, such as 7,000 psi, to sufficiently high levels at flaw sites to cause fracture initiation of the glass structure. Once fracture is initiated, it can propagate catastrophically through the glass structure because glass is a brittle material and high local stresses are not relieved by plastic flow.
Work in the field of the mechanics of glass structures has established that glass fracture initiation starts almost exclusively at surface defects. The characteristics of these defects, including their geometry, depth of penetration, orientation relative to the surface, etc. determine the extent to which they magnify an average applied tensile stress. Because glass is known to fail under tensile stress, the extent of magnification of applied stress largely determines the observed nominal strength of a glass object.
In order to increase the mechanical service strength of glass objects to a higher proportion of the theoretical strength, two basic avenues have conventionally been pursued. One approach is to attempt the production of glass objects having minimal surface defects, or with defects of a type which cause the least magnification of nominal applied tensile stresses. According to this approach, very high strength levels have been achieved by the application of chemical reagents or solvents to remove from the object the surface margin which contains the flaws, defects and other imperfections. Tensile strength levels up to several hundred thousand psi have been reported with this approach using a number of reagents, most commonly HF solutions. However, a major limitation of this approach arises from its low productivity and high cost. In order to achieve the desired strength improvement it may be necessary to remove as much as 2 mils of glass. The hazards of working with potent chemical reagents such as HF and the problems of disposal thereof are further deterrents to the practical commercial implementation of this approach.
The second major approach to improving the service strength of glass objects is to create residual compressive stress in the surface zone or skin of the glass. The purpose of such a residual compression zone is to place the imperfections under compression. In such circumstance, the defects can initiate fracture only when they are subjected to sufficient levels of tensile stress to overcome the residual compressive stress and reach the tensile stress levels at which fracture propagation can occur. The net practical effect is that the observed service tensile strength of the glass is increased by the magnitude of the residual compressive stress.
There are a number of known techniques for creating residual compressive stress in glass. Among the most practical are thermal tempering, application of case glass, and strengthening by ion exchange. Thermal tempering is normally effective only on thicknesses greater than about 1/8 inch, making it suitable for some applications but not satisfactory for the strengthening of glass objects such as containers for carbonated beverages. Tempering is an especially unpromising alternative for beverage bottles since the industry is continuing to move in the direction of lighter weight non-returnable bottles. In the case glass method, a layer of glass is bonded to a surface of a glass object to provide a region that is under residual compressive stress. Such an approach is complicated and expensive, and not well adapted to mass production operations such as the manufacture of glass beverage bottles.
One of the more attractive alternatives for providing a layer of residual compressive stress is ion exchange. In accordance with this technique the outer margin of the glass is reacted with a salt whose cations have ionic diameters different from the principal cations of the glass. Where the ion exchange salt contains larger cations, the compressive stress is directly produced and the process is referred to as ion stuffing. Where smaller diameter cations are used, a marginal stratum is generated having a lower coefficient of thermal expansion than the bulk of the glass so that, on cooling from the temperature at which the exchange reaction is conducted, the outer margin is placed under compressive stress.
Despite its demonstrated effectiveness, the ion exchange process has not found widespread application in the manufacture of beverage bottles. The major problem is the handling of the ion exchange salt. This material must be applied to the outside of the container in a molten bath, an aqueous solution spray, or as an air conveyed dust. Each of these techniques involves significant capital investment, operating and maintenance costs. Additionally, residual ion exchange material and ion exchange reaction products adhere to the outside of the container or other glass object after the reaction step is complete. This material must be removed in a separate washing step, and either discarded or recovered and recycled for treatment of additional glass. Such recovery and recycle facilities add to the expense. Also even where a recycle operation is carried out, a purge stream is required to dispose of products of the ion exchange reaction. This brings environmental considerations into play and may require further expense for disposal facilities.
An unfulfilled need has remained in the art, therefore, for an improved method for the clean, economical production of high service strength glass objects, most particularly glass bottles.